The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Residual currents caused by the failure of insulation can constitute a significant risk to safety in electrical systems. Using an appropriate protective concept it is possible to detect residual currents, discover/eliminate insulation faults quickly, and therefore ensure the availability of the system.
The acronym “RCM” stands for “Residual Current Monitoring” and means the monitoring of residual currents in electrical systems. While the current sensors in the electrical systems referred to herein are AC current sensors, if different current sensor types or configurations are used, DC components could also be accurately measured using the same monitoring method. The residual current is calculated as the non-zero sum of the currents flowing through all current-carrying conductors, apart from the protective earth (PE), which feed into the electrical system. Residual currents are typically the result of insulation faults or electromagnetic compatibility (EMC) filter component faults in a power supply, for example. While RCD devices (residual current circuit breakers) switch off the power supply in the event of a certain residual current being exceeded, RCM measuring devices indicate the actual value, record the long-term development and report when the measured value exceeds a critical value. This information can also be used in order to switch off the power supply via external switching devices (contactors, relays). Through the use of residual current measuring (RCM) devices, it is possible to detect and report residual currents in a timely manner. This makes it possible to initiate counter measures within a sufficiently short time so that it is not necessary to switch the system off. This facilitates the implementation of measures in the event of slowly deteriorating insulation values or steadily rising residual currents—caused for example by aging insulation—before the system is switched off.
A power strip, often referred to as a “power distribution unit” (“PDU”), is typically used in a data center environment, with one or more units installed in racks arranged in rows, to power Internet Technology equipment (ITE). A single PDU may provide power to dozens of devices per rack via outlet sockets, and an “intelligent” power strip or rack PDU employed in a data center can measure and control the loads. A powered device, e.g., server or network switch, has one or more internal switched mode power supplies that on occasion may fail prematurely for various reasons, e.g., exposure to excessively high-temperature which can degrade electronic component lifetime. Also, the integrity of the TNS (Terra Neutral Separate) earthing systems may become inadvertently disconnected or fail. These failures may develop suddenly or gradually over time. The failure mode may result from compromised or complete breakdown of conductor or component insulation spacings, resulting in a lower impedance conduction path between line voltages of the device's power supply to protective earth ground. While the residual current that flows through the protective earth ground is not of sufficient magnitude to trip the unit's branch overcurrent protection device, only 30 mA can become a safety hazard to anyone touching the chassis. While it is critical to identify and provide an alert when a condition of excessive ground residual current develops, it is also important that the offending device can be quickly isolated and removed from the power distribution to maintain high-availability of other systems components.
One particular known technique for measuring residual current involves using a sensitive current transformer to detect and/or measure the residual current or current that does not flow back on the return path through an intelligent power strip. Although this method is able to detect that a residual current condition exists, it is unable to identify which one of a plurality of voltages of a multiphase voltage supply is sourcing the residual current condition, as well as which specific AC outlet of the PDU is associated with the residual current condition.